1. Field of the Invention
The present invention relates to magnetic recording and reproducing apparatuses. More particularly, the present invention relates to a magnetic recording and reproducing apparatus using a microwave assisted recording method.
2. Description of the Related Art
Improvement of the performance of magnetic recording media and magnetic heads is desirable in hard disk apparatuses in order to increase magnetic recording densities. In the increase of the recording densities in the magnetic recording media, it is necessary to decrease the size of magnetic particles composing recording layers of the magnetic recording media in order to ensure signal to noise ratios (SNRs) desired for reproduction. However, the decrease in the size of the magnetic particles reduces the volume of the magnetic particles and thus disappearance of magnetization caused by thermal fluctuation may occur more frequently.
In order to prevent the disappearance of magnetization caused by the thermal fluctuation and to keep a stable recording state, it is desirable to increase magnetic anisotropy energy (hereinafter sometimes simply referred to as Ku) of the magnetic particles composing the recording layers of the magnetic recording media. The magnitude of the magnetic field to reverse the magnetization in the magnetic particles having uniaxial magnetic anisotropy is called magnetic anisotropy field Hk and is represented by Hk=2Ku/Ms where Ms denotes saturation magnetization. Accordingly, when a material having a high Ku is used, the magnetic anisotropy field Hk is increased and a strong recording magnetic field may be required for the recording on the magnetic recording media.
In contrast, since the strength of the magnetic fields generated by the magnetic heads is restricted by the materials and the shapes of the magnetic heads, it may be difficult to perform the recording in some cases.
In order to address such technical problems, exchange coupled composite (ECC) media are being considered. The ECC medium has magnetic layers of different magnetic properties, that is, a high Ku magnetic layer and a low Ku magnetic layer. In the ECC medium, the coupling force between the magnetic layers is controlled to reduce the recording reversed magnetic field of the magnetic recording medium while keeping thermal stability (refer to Japanese Unexamined Patent Application Publication No. 2005-222675 and Japanese Unexamined Patent Application Publication No. 2011-113604).
In addition, energy assisted recording is proposed in which energy is supplementarily applied to the magnetic recording media during the recording, thereby causing effective recording magnetic field strength to be reduced. A recording method using a microwave magnetic field as an auxiliary energy source is called microwave assisted magnetic recording (MAMR). Research and development of the MAMR is advanced for practical use.
Examples of the microwave assisted magnetic recording method include a method in which a spin torque oscillator (STO) composed of magnetic thin films of multiple layers is formed in a magnetic gap between a main magnetic pole composing a recording element of the magnetic head and a trailing shield, the microwave magnetic field in an in-plane direction is generated by self oscillating of the STO, and the microwave magnetic field is applied to the magnetic recording medium to induce magnetization precession, thereby assisting the perpendicular magnetization reversal. Specifically, a field generating layer (FGL) of the STO composed of the magnetic thin films of multiple layers is subjected to the self oscillating at a high frequency and a leaked magnetic field generating from the surface of the FGL is applied to the magnetic recording medium, thereby assisting the magnetization reversal with the microwave. This assist method is called a self oscillating microwave assisted magnetic recording method (refer to Japanese Unexamined Patent Application Publication No. 2009-064487).
Another example of the microwave assisted magnetic recording method other than the self oscillating microwave assisted magnetic recording method is an externally oscillated microwave assisted magnetic recording method. An apparatus using the method has been devised in which a strip line is arranged near the main magnetic pole of the magnetic head, high-frequency current in a microwave band is externally passed through the strip line to generate a high-frequency in-plane magnetic field near the main magnetic pole, and the high-frequency in-plan magnetic field is superposed on the perpendicular recording magnetic field generated by the main magnetic pole to assist the magnetization reversal (refer to Japanese Unexamined Patent Application Publication No. 2007-299460).
In order to increase the recording density, it is necessary to reduce the bit length (magnetization reversal interval) of a recording signal and narrow down a recording track width. The possibility of reducing the magnetic field strength to be used for the magnetization reversal of a high magnetic anisotropy energy medium that is microparticulated by using a combination of the ECC medium and the microwave assisted magnetic recording method is known (refer to T. Tanaka, N. Narita, A. Kato, Y. Nozaki, Y. Hong, and K. Matsuyama, “Micormagnetic Study of Microwave-Assisted Magnetization Reversals of Exchange-Coupled Composite Nanopillars”, IEEE Transactions on Magnetics, vol 49, No. 1, 562 (2013)). However, although the reduction in the magnetic field to be used for the magnetization reversal enables the recording on the high magnetic anisotropy energy medium, spread of the recording in the track width direction may easily occur concurrently, which causes a problem in that improvement of the track density is difficult. There has not been found any useful means for realizing the narrow recording track width other than a method of decreasing the size of an element that generates the microwave so as to minimize the strength distribution of the microwave (refer to Y. Tang and J. G. Zhu, “Narrow track confinement by AC field generation layer in microwave assisted recording” IEEE Transactions on Magnetics, vol 44 No. 11 3376 (2008)).
Specifically, in the externally oscillated microwave assisted magnetic recording method using the microwave generated by a microstrip line arranged near the main magnetic pole of the magnetic head, preferable specifications of the recording layer of the ECC medium that concurrently realizes high SNR and the narrow recording track width and the method of applying the microwave are not clear.